The present invention relates to an ion source for stably generating a well-focused ion beam of, especially, multicharged ions.
Two ways have been proposed in the art for generating multiply-charged ions. Electrons included in an atom are successively stripped off in one method, whereby the number of the valence is increased one by one, namely, in the order of one-charged, two-charged, . . . , seven-charged, and eight-charged, . . . , whereas a plurality of electrons are torn off at one time in another method.
The latter method is not effective since the creation of the multicharged ions is of low probability. It is of great importance in the former method that ionized particles come into collision with electrons of high velocity before they come into collision with uncharged particles in order to produce multiply-charged ions. It is, therefore, required that the particles have a long mean free path and the ionization is performed in a considerably high vacuum. The conventional ion source did not fulfill the above requirements, because the conventional ion source was operable under the gas pressure of 10.sup.-.sup.2 - 10.sup.-.sup.4 Torr. The ionization must be carried out under the gas pressure of 10.sup.-.sup.6 - 10.sup.-.sup.10 Torr. That is, the gas pressure suitable for the normal glow discharge and the arc discharge is not available.